Self-adjustment and disintegration threshold of Langmuir solitons in inhomogeneous plasmas

Abstract

Propagation of Langmuir solitons in inhomogeneous plasmas is investigated numerically. Through numerical simulation solving Zakharov equations, the solitons are accelerated toward the low density side. As a consequence, isolated cavities moving at ion sound velocities are emitted. When the acceleration is further increased, solitons collapse and the cavities separate into two lumps released at ion sound velocities. The threshold is estimated by an analogy between the soliton and a particle overcoming the self-generated potential well.PACS numbers: 52.65.Cc, 52.35. Kt, 52.55.Pi Langmuir solitons were theoretically predicted by Zakharov [1]. In a homogeneous medium, the Langmuir solitons can propagate without changing their form when the non-linearity from a ponderomotive force and dispersion balance exactly. Following the theoretical prediction, Langmuir solitons were observed in a laboratory experiment via the trapping of density cavities by imposing an external radio frequency (RF) electric field [2]. In the experiment, the external electromagnetic waves underwent mode conversion to become electrostatic waves [3]. It should be noted that the Langmuir soliton is also referred to as a caviton, and consists of one electric field soliton and one density cavity.Seen from a practical application point of view, the generation and collapse of Langmuir solitons are closely related to space weather forecasts. Langmuir turbulence driven by oscillating two stream instabilities is caused by the acceleration of electrons via the localized electric field of Langmuir solitons [4,5]. More specifically, type III solar bursts induce Langmuir turbulence [6], which in turn emit radio waves with frequencies at higher harmonics that reach earth in eight minutes and act as precursors for geomagnetic storms. Such storms typically arrive a few days later. Understanding Langmuir solitons and Langmuir turbulence is one of the central issues for the space weather forecast program.In this paper, we first investigate whether Langmuir solitons can stably propagate in inhomogeneous media. We then examine the collapse mechanism of the solitons and discuss the threshold for the collapse. The acceleration of Langmuir solitons in inhomogeneous media is studied analytically employing the nonlinear Schrödinger equation (NLSE) [7] and Zakharov equations [8] in a small acceleration limit. Nevertheless, the dynamics of the solitons at large acceleration and their survival threshold remains a question to be clarified. In this work, * Electronic address: nishimura@pssc.ncku.edu.tw we employ a set of nonlinear fluid equations, the Zakharov equations to investigate the acceleration of Langmuir solitons with inhomogeneous plasma background densities by numerical simulation. Based on the findings, we introduce the idea of quasi-particles, through which we interpret the acceleration as an analogy of a point mass falling down the potential well in comparison with the Shrödinger equation in quantum mechanics.As elaborated later, we discovered a by-product of...